Catalyst slurry preparation system and use thereof

ABSTRACT

The present invention relates to the use of a catalyst preparation system for the preparation of a diluted catalyst slurry. In particular, the invention relates to a catalyst preparation system comprising a mixing vessel for mixing a particulate catalyst and a liquid hydrocarbon diluent. According to the invention, diluted catalyst slurry is prepared in a mixing vessel comprising a rotatable axial impeller system comprising at least two double-bladed hubs. The invention also relates to a process for preparing diluted catalyst slurry for use in the preparation of a particulate polyethylene product in a loop reactor with the catalyst preparation system as described herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT/EP2011/063136, filed Jul. 29,2011, which claims priority from EP 10171370.9, filed Jul. 30, 2010.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the use of a catalyst preparationsystem for preparing a diluted catalyst slurry which is to be used forproducing a particulate polyethylene product in a loop reactor. Inparticular, the invention relates to such use, wherein the systemcomprising a mixing vessel for mixing a particulate catalyst and aliquid hydrocarbon diluent. According to the invention, diluted catalystslurry is prepared in a mixing vessel comprising a rotatable impellersystem comprising double-bladed hubs.

BACKGROUND OF THE INVENTION

Polyethylene (PE) is synthesized by polymerizing ethylene (CH₂═CH₂)monomers. Because it is cheap, safe, stable to most environments andeasy to be processed polyethylene polymers are useful in manyapplications. According to the properties polyethylene can be classifiedinto several types, such as but not limited to LDPE (Low DensityPolyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (HighDensity Polyethylene). Each type of polyethylene has differentproperties and characteristics.

Ethylene polymerizations are frequently carried out in a loop reactorusing ethylene monomer, liquid diluent and catalyst, optionally one ormore co-monomers, and hydrogen. The polymerization in a loop reactor isusually performed under slurry conditions, with the produced polymerusually in a form of solid particles which are suspended in the diluent.The slurry in the reactor is circulated continuously with a pump tomaintain efficient suspension of the polymer solid particles in theliquid diluent. Polymer slurry is discharged from the loop reactor bymeans of settling legs, which operate on a batch principle to recoverthe slurry. Settling in the legs is used to increase the solidsconcentration of the slurry finally recovered as product slurry. Theproduct slurry is further discharged through heated flash lines to aflash tank, where most of the diluent and unreacted monomers are flashedoff and recycled.

Alternatively, the product slurry may be fed to a second loop reactorserially connected to the first loop reactor wherein a second polymerfraction may be produced. Typically, when two reactors in series areemployed in this manner, the resultant polymer product is a bimodalpolymer product, which comprises a first polymer fraction produced inthe first reactor and a second polymer fraction produced in the secondreactor, and has a bimodal molecular weight distribution.

After the polymer product is collected from the reactor and thehydrocarbon residues are removed therefrom, the polymer product isdried, additives can be added and finally the polymer may be extrudedand pelletized.

During the extrusion process ingredients including polymer product,optional additives, etc, are mixed intimately in order to obtain acompound as homogeneous as possible. Usually, this mixing is done in anextruder wherein the ingredients are mixed together and the polymerproduct and optionally some of the additives are melted so that intimatemixing can occur. The melt is then extruded into a rod, cooled andgranulated, e.g. to form pellets. In this form the resulting compoundcan then be used for the manufacturing of different objects.

Polymerization of ethylene involves the polymerization of ethylenemonomer in the reactor in the presence of a polymerization catalyst andoptionally, if required depending on the used catalyst, an activatingagent. Suitable catalysts for the preparation of polyethylene comprisechromium catalysts, Ziegler-Natta catalysts and metallocene catalysts.Typically, the catalyst is used in particulate form. The polyethylene isproduced as a resin/powder with a hard catalyst particle at the core ofeach grain of the powder.

Several systems have been disclosed which involve the preparation andthe supply of catalyst slurry to a polymerization reaction. In general,for preparing catalyst slurry, a mixture of dry solid particulatecatalyst and diluent are apportioned in a catalyst mixing vessel andthoroughly mixed. Then such catalyst slurry is typically transferred toa polymerization reactor for contact with the monomer reactants.

It is known in the art that for the production of ethylene polymershaving suitable properties it is important during polymerization tocontrol reaction conditions, including reaction temperatures, reactantconcentration, etc. Polymerization reactions are also sensitive to thequantity, quality and the type of catalyst utilized. Sub-optimalconditions at the start of or during the polymerization reaction maylead to a sub-optimal polymerization conditions resulting for instancein low production yields and/or the production of polymers havingundesired properties and/or falling off specifications. In view thereof,ethylene polymerization reactions require accurate and adaptivemonitoring and control of the reaction conditions.

In particular, the concentration of a particulate catalyst in a diluenthas a direct and immediate effect on polymer characteristics such aspolymerization product granulometry and polymerization product particledensity, as well as on polymerization characteristics such aspolymerization processivity. Therefore, a change in catalystconcentration has a profound effect on various polymerization parametersand hence the final polymer product. Indeed, (local) catalystconcentration differences in a polymerization reaction result inunwarranted polymer heterogeneity in respect of for instance productdensity, granulometry and molecular weight (distribution).

As catalyst slurries comprise a solid particulate catalyst suspended ina liquid diluent, such slurries are prone to sedimentation. Adequatemixing of the catalyst slurry is needed to assure a homogeneousdistribution of the solid catalyst particles in the diluent before thecatalyst slurry is fed to the polymerization reactor.

Moreover, the physicochemical characteristics of the catalyst well as ofthe diluent, including for instance the type of catalyst and diluent,the specific gravity of the catalyst, catalyst granulometry, catalystsettling velocity, catalyst concentration, the viscosity of the diluentand the catalyst slurry as well as the desired properties of thepolymerization product require a highly flexible and adaptable catalystslurry preparation and mixing system for adequately preparing a dilutedcatalyst slurry.

In view of the above, there remains a need in the art to provide animproved catalyst preparation system for preparing a diluted catalystslurry with suitable properties for use in a polymerization process formaking polyolefin resin, and in particular polyethylene.

SUMMARY OF THE INVENTION

The present invention relates to the use of a catalyst slurrypreparation system for the preparation of a diluted catalyst slurrycomprising a solid particulate catalyst and a liquid hydrocarbondiluent, wherein said catalyst slurry preparation system comprises acylindrical mixing vessel wherein said mixing vessel comprises a toppart, a bottom part and a rotatable impeller system which is actuated bya motor, said impeller system comprising a magnetic actuated agitatorshaft which is positioned along a longitudinal axis of said mixingvessel and extends through said top part of said mixing vessel andcomprises at least two double-bladed hubs, which are fixed to saidagitator shaft.

The present invention also relates to the use of the catalyst slurrypreparation system as described above for the preparation of a dilutedcatalyst slurry starting from settled catalyst. In particular, thepresent invention also relates to the use of the catalyst slurrypreparation system as described above for suspending or resuspendingsettled catalyst in the mixing vessel. As used herein, “settledcatalyst” is meant catalyst or catalyst particles, e.g. catalyst whichis provided onto a carrier, which has precipitated or undergonesedimentation and settled at the bottom of the mixing vessel, e.g. underthe influence of gravity, and which is thus not distributedhomogeneously anymore in the diluent.

The present invention further relates to a catalyst slurry preparationsystem as described above for the production of a catalyst slurry, inparticular a diluted catalyst slurry, to be used in an ethylenepolymerization process.

The present invention further relates to a method for preparing acatalyst slurry, in particular a diluted catalyst slurry, with thecatalyst slurry preparation system as described above.

The inventors have surprisingly found that in the context of catalystslurry preparation an impeller system comprising two or moredouble-bladed impellers mounted on a central axis provides for efficienthomogenization of the catalyst in the diluent. In this way, catalystslurry of controlled properties, in particular a controlled catalystconcentration can be fed to a polymerization reactor, resulting in apolymerization reaction, in particular ethylene polymerization, for theproduction of a polymer with uniform and homogeneous physicochemicalcharacteristics.

In an embodiment, the input of concentrated catalyst is performed usinga metering device and the diluent is fed by pressure regulated device.The output of diluted catalyst slurry from the mixing vessel to thepolymerization loop reactor typically is performed with a pump underhigh pressure. The catalyst slurry preparation system according to theinvention assures adequate mixing and homogenization of the catalystslurry under these conditions. In particular, slurry comprisingdifferent types of particulate catalysts, combinations of catalystslurries or catalysts with a varying particle size or a broaddistribution of particle size can be efficiently mixed in the catalystslurry preparation system according to the invention.

As used herein, “adequate mixing” comprises homogenization, or thecreation of a homogeneous suspension with a substantially consistentconcentration over time. Adequate mixing further means that no relevantsedimentation of the catalyst or catalyst provided onto a carrier takesplace during operation, i.e. the catalyst or catalyst provided onto acarrier remains in suspension during operation. Adequate mixing furthercomprises the resuspension of settled or precipitated catalyst orcatalyst provided onto a carrier.

The at least two impellers are fixed, preferably slidably fixed, on anagitation shaft, which is positioned along a central axis of the mixingvessel. Therefore, the position of the impellers, i.e. the double-bladedhubs, along the agitator shaft advantageously can be individuallyadjusted, depending on the physicochemical characteristics of thecatalyst and diluent to ensure optimal mixing and homogenization of thecatalyst slurry.

In an embodiment, the invention relates to the use as indicated above,wherein the mixing vessel of catalyst slurry preparation systemaccording to the invention further comprises one or more baffle(s),wherein said one or more baffles are fixed longitudinally along theinner wall of the mixing vessel, whereby said one or more baffles extendradially inward. The use of baffles prevents the movement of the fluidinside the mixing vessel as a whole (i.e. as one body) and thus preventsvortex movement. In contrast to vortex movement, the baffles assureturbulent movement, thereby helping in thorough mixing andhomogenization.

In an embodiment, the invention relates to the use as indicated above,wherein said one or more baffles extend radially inward over a distancebetween 10% and 20% of the diameter of the mixing vessel.

In an embodiment, the invention relates to the use as indicated above,wherein the blades of each double-bladed hub are symmetricallypositioned about the hub and have a pitch angle (α) comprised between65° and 75°. The pitch angle of the blades of the impellers according tothe invention assures the creation of an axial flow of the catalystslurry as well as a radial flow of the catalyst slurry. Whereas apredominant axial flow or radial flow would direct the catalyst slurryrespectively predominantly downwards or sidewards, a combined axial andradial flow effectively combines flow directions, likewise aiding inthorough mixing and homogenization.

In another embodiment, the invention relates to the use as indicatedabove, wherein each blade of said double-bladed hub is an airfoilcomprising a root portion attached to the hub and a radially outwardlydisposed tip portion with therebetween an upper side edge facing the toppart of said mixing vessel and a lower side edge facing said bottom partof said mixing vessel. It has been found that specifically double-bladedimpellers are most useful in preparing homogeneous catalyst slurriesaccording to the invention.

In an embodiment, the invention relates to the use as indicated above,wherein the tip portion of the blades according to the inventioninclines convexly toward the top part of the mixing vessel beyond theupper side edge and extends more proximal to the top part of the mixingvessel than the root portion, wherein the upper side edge extendslaterally from the root portion to the tip portion and inclines convexlytoward the tip portion, wherein the lower side edge is concave. Thisspecific shape of the blades of the impeller according to the inventionhas been found to accord perfectly with the desired level of mixing andhomogenization of the catalyst slurry.

In another embodiment, the invention relates to the use as indicatedabove, wherein a first double-bladed hub is fixed to the agitator shaftin the lower half of the mixing vessel.

In yet another embodiment, the invention relates to the use as indicatedabove, wherein a second double-bladed hub is fixed to the agitator shaftin the lower quarter of the mixing vessel.

In yet another embodiment, the invention relates to the use as indicatedabove, wherein the distance between the first and second double-bladedhub is between one half to one third of the length of the agitatorshaft.

Depending on the specific application, the position of the at least twoimpellers can be varied with respect to the dimensions of the mixingvessel and with respect to each other as well as in view of the fillinglevel of the mixing vessel. The versatility of arrangement of theimpellers in the mixing vessel allows for a swift, and hence economical,adjustment of the catalyst slurry preparation system, allowing easychange of for instance catalyst or diluent type with varying catalystconcentration and/or slurry viscosity.

In an embodiment, the invention relates to the use as indicated above,wherein the blade span of each double-bladed hub is between 30% and 50%of the diameter of the mixing vessel. The dimensions of the impellersaccording to the invention have likewise been found to aid in an optimalmixing and homogenization of the catalyst slurry.

According to the invention, the invention relates to the use asindicated above, wherein the at least two double-bladed hubs direct theflow to the bottom part of the mixing vessel. The impellers are mountedabout the agitator shaft in such way, that each impeller directs theflow downward, i.e. away from the drive, away from the catalyst anddiluent inlets and towards the catalyst slurry outlet. In sucharrangement, when feeding the catalyst slurry to a downstreampolymerization reactor, contradicting forces are minimized.

In an embodiment, the invention relates to the use as indicated above,wherein the impeller system is a magnetic drive impeller system, whereinthe motor is an electric motor which drives an adjustable magneticcoupling to transfer torque to the agitation shaft. A magneticallydriven agitator has the advantage that the impeller system which itcomprises is set in motion by a magnetic coupling which occurs withoutphysical contact between two rotating parts of which one is driven bythe driven shaft of an electric motor while the other is constituted bya propelling screw or shaft. This makes it possible to arrange the partassociated with the shaft of the electric motor outside the recipientwhile the propelling screw is installed inside the recipient. Any dangerof leakage at the level of the agitator may thus be set aside. As aresult, the mixing tank can be operated liquid full, without any risk ofleakage and avoids environmental and safety hazards. This isparticularly useful when the mixture is toxic or when pollution thereofby outside agents is to be avoided, such as for example in the case ofcatalyst preparations under pressurized conditions.

In an embodiment, the invention relates to the use as indicated above,wherein said catalyst slurry preparation system further comprises one ormore mud pots suitable for containing concentrated catalyst slurry,wherein each mud pot is operably connected to said mixing vessel.

In another embodiment, the invention relates to the use as indicatedabove, wherein said mixing vessel is a liquid full tank.

In yet another embodiment, the invention relates to the use as indicatedabove, wherein the concentration of said solid particulate catalyst insaid liquid hydrocarbon diluent is preferably between 0.1% and 10% byweight, for instance between 0.5 and 5% by weight, for example between0.3 and 3% by weight, more preferably at least 0.2% by weight, and mostpreferably at least 0.3% by weight, and more preferably at most 5%, andmost preferably at most 3% by weight.

The invention relates to the use as indicated above, wherein said solidparticulate catalyst has an average diameter comprised between 1 μm and100 μm, preferably between 5 μm and 100 μm, more preferably between 5 μmand 50 μm, and most preferably between 15 and 50 μm.

In another embodiment, the invention also relates to the use asindicated above, wherein said impeller system is rotatable at a speed ofbetween 50 and 1000 rpm, and preferably between 150 and 450 rpm.

The invention also relates to the use of the catalyst slurry preparationsystem according to the invention for the preparation of a catalystslurry comprising a solid particulate catalyst and a liquid hydrocarbondiluent, the solid particulate catalyst comprising an inert particle onwhich a catalyst selected from the group comprising metallocenecatalysts, Ziegler-Natta catalysts and chromium catalysts isimmobilized.

These and further aspects and embodiments of the invention are furtherexplained in the following sections and in the claims, as well asillustrated by non-limiting figures.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic overview of an embodiment of a catalyst slurrypreparation system according to the invention.

FIG. 2 depicts an embodiment of a mixing vessel of a catalyst slurrypreparation system according to the invention.

FIG. 3 is a detailed view of an embodiment of a double-bladed hub fixedon an agitator shaft that can be applied in a mixing vessel according tothe invention.

FIG. 4 illustrates the pitch angle (α) of a blade of a double-bladed hubfixed on an agitator shaft.

FIG. 5 illustrates the concentration of catalyst issued from a catalystmixing vessel and prepared using a catalyst slurry preparation systemaccording to the invention over a period of time of 2 days.

FIG. 6 represents a schematic cross-view of some elements of therotatable magnetic drive impeller system 25.

DETAILED DESCRIPTION OF THE INVENTION

Before the present method and products of the invention are described,it is to be understood that this invention is not limited to particularmethods, components, products or combinations described, as suchmethods, components, products and combinations may, of course, vary. Itis also to be understood that the terminology used herein is notintended to be limiting, since the scope of the present invention willbe limited only by the appended claims.

As used herein, the singular forms “a”, “an”, and “the” include bothsingular and plural referents unless the context clearly dictatesotherwise.

The terms “comprising”, “comprises” and “comprised of” as used hereinare synonymous with “including”, “includes” or “containing”, “contains”,and are inclusive or open-ended and do not exclude additional,non-recited members, elements or method steps. It will be appreciatedthat the terms “comprising”, “comprises” and “comprised of” as usedherein comprise the terms “consisting of”, “consists” and “consists of”.

The recitation of numerical ranges by endpoints includes all numbers andfractions subsumed within the respective ranges, as well as the recitedendpoints.

The term “about” or “approximately” as used herein when referring to ameasurable value such as a parameter, an amount, a temporal duration,and the like, is meant to encompass variations of +/−10% or less,preferably +/−5% or less, more preferably +/−1% or less, and still morepreferably +/−0.1% or less of and from the specified value, insofar suchvariations are appropriate to perform in the disclosed invention. It isto be understood that the value to which the modifier “about” refers isitself also specifically, and preferably, disclosed.

All documents cited in the present specification are hereby incorporatedby reference in their entirety.

Unless otherwise defined, all terms used in disclosing the invention,including technical and scientific terms, have the meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. By means of further guidance, term definitions are included tobetter appreciate the teaching of the present invention.

In the following passages, different aspects of the invention aredefined in more detail. Each aspect so defined may be combined with anyother aspect or aspects unless clearly indicated to the contrary. Inparticular, any feature indicated as being preferred or advantageous maybe combined with any other feature or features indicated as beingpreferred or advantageous.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to a person skilled in the art from this disclosure, in one ormore embodiments. Furthermore, while some embodiments described hereininclude some but not other features included in other embodiments,combinations of features of different embodiments are meant to be withinthe scope of the invention, and form different embodiments, as would beunderstood by those in the art. For example, in the following claims,any of the claimed embodiments can be used in any combination.

In an aspect, the invention relates to the use of a catalyst slurrypreparation system for preparing a catalyst slurry, in particular adiluted catalyst slurry, wherein said catalyst slurry preparation systemcomprises a mixing vessel comprising a top part, a bottom part and arotatable impeller system which is actuated by a motor, the impellersystem comprising a magnetic actuated agitator shaft which is positionedalong a longitudinal axis of said mixing vessel and extends through thetop part of the mixing vessel and comprises at least two double-bladedhubs, which are fixed to the agitator shaft. In an embodiment, thedouble-bladed hubs are fixed to the agitator shaft.

In an embodiment, one or more mud pots, suitable for containingconcentrated catalyst slurry, are operably connected to the mixingvessel, preferably by means of one or more conduits, as for instanceillustrated on FIG. 2.

As used herein, the term “mixing vessel” is meant to comprise any kindof closed recipient suitable for use in mixing substances, in particularpressurized substances, such as catalyst slurries that are used in thepreparation of polyethylene product in a loop reactor. According to theinvention, the mixing vessel is a cylindrical vessel. In an embodiment,the height to diameter ratio of the mixing vessel is comprised between1.5 and 2.5, and preferably is about 2.

In an embodiment, the mixing vessel is a liquid full tank, which meansthat in operation, the vessel is completely filled with catalyst slurryand has no or substantially no gas void. The term “essentially free of agaseous phase” therefore refers to a state of the mixing vessel whereinmaximal amount of gas in the mixing vessel 3 is at most 5% of the vesselvolume, preferably at most 4%, at most 3%, at most 2%, at most 1% or atmost 0.5% and most preferably at most 0.1% of the vessel volume. Theliquid full state of the mixing vessel refers to a situation wherein thetotal wetted surface would be 95% or more of the total inner surface ofthe mixing vessel, preferably 96%, 97%, 98%, 99%, 99.5%, 99.9% or moreof the total inner surface of the mixing vessel and most preferably 100%of the total inner surface of the mixing vessel. As used herein, “wettedsurface” is the inner surface of the mixing vessel which is in directcontact with the liquid inside the vessel, i.e. the catalyst slurry.

In another embodiment, the volume of the mixing vessel is at least 200 l(liter), preferably at least 400 l, more preferably at least 450 l, andmost preferably at most 2000 l, more preferably at most 1000 l, mostpreferably at most 600 l and in particularly preferred at most 550 l,for instance about 500 l.

As used herein, the term “catalyst slurry” refers to a compositioncomprising catalyst solid particles, i.e. a solid or particulatecatalyst, and a diluent. The solid particles can be suspended in thediluent, either spontaneously or, according to the invention, byhomogenization techniques, such as mixing. In the present invention itis especially applicable to solid particles of ethylene polymerizationcatalyst in a liquid diluent. These slurries will be referred to hereinas ethylene polymerization catalyst slurries.

By the term “solid particles” it is meant a solid provided as acollection of particles, such as for instance a powder or granulate. Inthe present invention it is especially applicable to a catalyst providedon a carrier or support. The support is preferably a silica (Si)support.

As used herein, the “catalyst” refers to a substance that causes achange in the rate of a polymerization reaction without itself beingconsumed in the reaction. In the present invention it is especiallyapplicable to catalysts suitable for the polymerization of ethylene topolyethylene. These catalysts will be referred to as ethylenepolymerization catalysts. In the present invention it is especiallyapplicable to ethylene polymerization catalysts such as metallocenecatalysts, chromium catalysts and/or Ziegler-Natty catalysts. Whereas“catalyst slurry” refers herein to a composition comprising catalystsolid particles and a diluent, “catalyst” refers herein to the catalystmolecules either as such or provided on a carrier or support.

Catalyst slurry according to the invention consists or consistsessentially of solid catalyst, such as those given above, and a liquidhydrocarbon diluent. Diluents which are suitable for being used inaccordance with the present may comprise but are not limited tohydrocarbon diluents such as aliphatic, cycloaliphatic and aromatichydrocarbon solvents, or halogenated versions of such solvents. Thepreferred solvents are C₁₂ or lower, straight chain or branched chain,saturated hydrocarbons, C₅ to C₉ saturated alicyclic or aromatichydrocarbons or C₂ to C₆ halogenated hydrocarbons. Non-limitingillustrative examples of solvents are butane, isobutane, pentane,hexane, heptane, cyclopentane, cyclohexane, cycloheptane, methylcyclopentane, methyl cyclohexane, isooctane, benzene, toluene, xylene,chloroform, chlorobenzenes, tetrachloroethylene, dichloroethane andtrichloroethane. In a preferred embodiment of the present invention,said diluent is isobutane. However, it should be clear from the presentinvention that other diluents may as well be applied according to thepresent invention.

As used herein the term “catalyst slurry preparation system” refers to adevice or system wherein catalyst slurry as defined herein is prepared.Said catalyst preparation system in connected to a polymerization loopreactor for supply of the prepared catalyst slurry to the reactor. In anembodiment, according to the invention, the catalyst slurry preparationsystem comprises at least one mixing vessel wherein catalyst slurry isdiluted to a concentration suitable for use in a polymerizationreaction; one or more mud pots containing concentrated catalyst slurry,one or more conduits connecting the one or more mud pots to the mixingvessel for transferring catalyst slurry from the one or more mud pots tothe mixing vessel, and one or more conduits connecting the mixing vesselto a polymerization reactor for transferring the diluted catalyst slurryto the polymerization reactor. The latter conduits may be provided withpumping means for pumping the catalyst slurry from the mixing vessel toa polymerization reactor.

As used herein, the term “concentrated catalyst slurry” refers to acomposition comprising catalyst solid particles that are in suspensionwhereby the concentration of catalyst is at least higher than 10% byweight. The term “diluted catalyst slurry” refers to a compositioncomprising catalyst solid particles that are in suspension, whereby theconcentration of catalyst is lower than or equal to 10% by weight, forexample between 0.1% and 10%, for example between 0.2 and 5% by weight,and for instance between 0.3 and 3%.

As used herein, the term “mud pot” refers to a storage vessel forconcentrated catalyst slurry in a hydrocarbon diluent. The catalystconcentration in a mud pot is higher than the catalyst concentration inthe mixing vessel. Therefore, after transfer of the concentratedcatalyst slurry from the mud pot to the mixing vessel, additionaldiluent is added in the mixing vessel.

As used herein, the term “impeller system” refers to a rotor fortransmitting motion. The impeller system is a system for mixing slurry,in particular slurry comprising a solid particulate catalyst and adiluent, in a mixing vessel. An impeller system according to theinvention comprises an agitator shaft connected to a motor. The motordrives rotation of the agitator shaft. Fixed on the agitator shaft areimpellers, which essentially are blades or vanes that are fixed onto ahub. This hub is fixed onto the agitator shaft of the impeller system.In an embodiment, the hub is slidably fixed onto the agitator shaft.This means that according to the needs, depending on e.g. catalystconcentration, viscosity, dimensions of the mixing vessel, the impellerscan be fixed onto the agitator shaft at specific positions.

According to the invention, the impeller system comprises an agitatorshaft and at least two double-bladed hubs fixed thereon, preferablyslidably fixed. In an embodiment, the impeller system comprises 2, 3, 4,5, or more double-bladed hubs. In a preferred embodiment, the impellersystem comprises two double-bladed hubs. As used herein, the term“double-bladed hub” means a hub comprising two blades. In an embodiment,the blades are positioned symmetrically onto the hub.

In an embodiment, the agitator shaft is positioned in the centre of themixing vessel, i.e. along the longitudinal central axis of the mixingvessel. In another embodiment, the agitator shaft is positioned at anoffset from the longitudinal central axis of the mixing vessel, i.e. theagitator shaft is positioned longitudinally in the mixing vessel but notlocated in the centre of the mixing vessel.

In an embodiment, the impeller system is a magnetic drive impellersystem, wherein the motor is an electric motor (electromotor) whichdrives an adjustable magnetic coupling to transfer torque to a magneticactuated agitation shaft. Preferably, the magnetic coupling is mounteddirectly on top of the mixing vessel, without intermediate joints. In anexample, an intank flange connection secures the agitator shaft to themixing vessel.

In an embodiment, each of the blades of each double-bladed hub is anairfoil. In a further embodiment, each of the blades of eachdouble-bladed hub is a flat plate airfoil. As used herein, the term“flat plate airfoil” refers to a blade which is essentially flat inappearance (i.e. viewed along a longitudinal axis from the tip of theblade to the root of the blade), with a constant thickness. In otherwords, the camber line of the blade is not curved, but instead is astraight line and the length of the camber line equals the length of thechord. As used herein, “chord” is the distance between the leading edgeof the blade and the trailing edge of the blade. As used herein, “camberline” is the line drawn midway between the upper and lower surface ofthe blade. The leading and trailing edge of a blade are respectively thefront and back of the blade in the direction of motion.

In an embodiment, each blade of each double-bladed hub comprises a rootportion attached to a hub and a radially outwardly disposed tip portionwith therebetween an upper side edge facing the top part of the mixingvessel and a lower side edge facing the bottom part of the mixingvessel. In a further embodiment, the tip portion inclines convexlytoward the top part of the mixing vessel beyond the upper side edge,wherein the upper side edge extends laterally from the root portion tothe tip portion and inclines concavely toward the tip portion, andwherein the lower side edge is concave. In an embodiment, the shape orform of the tip portion of the blade and the bottom part of the mixingvessel closely correspond. This similar shape assures that catalystparticles remain suspended also in the lower part of the mixing vessel.In an embodiment, the mean width of each blade—i.e. between upper sideedge and lower side edge—is about one third of the length of eachblade—i.e. between root portion and tip portion—. In an embodiment, theshape of each blade is as depicted in FIG. 3.

In an embodiment, the blade span of each double-bladed hub is betweenabout one third and one half of the diameter of the mixing vessel. In anembodiment, the blade span of each double-bladed hub is between about30% and 50% of the diameter of the mixing vessel. In another embodiment,the blade span of each double-bladed hub is between about 35% and 45% ofthe diameter of the mixing vessel. In a more preferred embodiment theblade span of each double-bladed hub is about 40% of the diameter of themixing vessel. As used herein, the term “blade span” is the distancebetween the tip portion of one blade of a double-bladed hub and the tipportion of the other opposing blade of that double-bladed hub.

In an embodiment, the distance between two double-bladed hubs which arefixed on the agitation shaft is between about one third and one half ofthe length (i.e. the height) of the mixing vessel. In anotherembodiment, the distance between two double-bladed hubs which are fixedon the agitation shaft is between about one third and one half of thelength of the agitation shaft. In an embodiment, the distance betweentwo double-bladed hubs is between about 30% and 50% of the height of themixing vessel. In a further embodiment, the distance between twodouble-bladed hubs is between about 35% and 45% of the height of themixing vessel. In a preferred embodiment, the distance between twodouble-bladed hubs is about 40% of the height of the mixing vessel.

In an embodiment, each double-bladed hub is fixed on the agitation shaftin the lower half of the mixing vessel. In another embodiment, the lowerdouble-bladed hub is fixed on the agitation shaft in the lower quarterof the mixing vessel. In a further embodiment, one double-bladed hub isfixed on the agitation shaft between about 70% and 50% of the length ofthe mixing vessel, starting from the top of the mixing vessel. Inanother embodiment, this double-bladed hub is fixed on the agitationshaft between about 65% and 55% of the length of the mixing vessel,starting from the top of the mixing vessel, more preferably about 60%.In an embodiment, a second double-bladed hub is fixed on the agitationshaft between about 95% and 75% of the length of the mixing vessel,starting from the top of the mixing vessel. In another embodiment, thisdouble-bladed hub is fixed on the agitation shaft between about 90% and80% of the length of the mixing vessel, starting from the top of themixing vessel, more preferably about 85%.

In an embodiment, the blades are symmetrically fixed on the hub and havea pitch angle (α) comprised between about 65° and 75°, preferably about70°. As used herein, the term “pitch angle” refers to the angle betweenthe chord of the blade and the rotational plane of the blade (in thedirection of rotation). The pitch angle can alternatively be referred toas the angle of attack.

In an embodiment, the mixing vessel of the catalyst slurry preparationsystem according to the invention further comprises one or morebaffle(s), wherein each baffle is fixed longitudinally along the innerwall of the mixing vessel, whereby the baffle(s) extend(s) radiallyinward. In an embodiment, the mixing vessel comprises 1, 2, 3, 4, 5, 6or more baffles. In a preferred embodiment, the mixing vessel comprisesthree baffles. In an embodiment, the baffles each extend along side wallof the mixing vessel for at least two-thirds of the length of the mixingvessel. As used herein, a “baffle” is essentially a flat screen for useto deflect or disrupt the flow in the mixing vessel.

In an embodiment, each baffle extends radially inward over a distance ofat least 5% of the diameter of the mixing vessel. In another embodiment,each baffle extends radially inward over a distance of between 5% and20% of the diameter of the mixing vessel. In a further embodiment, eachbaffle extends radially inward over a distance of between 10% and 20% ofthe diameter of the mixing vessel. In yet another embodiment, eachbaffle extends radially inward over a distance of between 5% and 15% ofthe diameter of the mixing vessel. In an embodiment, each baffle extendsradially inward over an equal distance.

In another aspect, the invention relates to a catalyst slurrypreparation system as described above.

Non-limiting examples of catalyst slurry preparation systems accordingto the present invention are for instance illustrated in FIGS. 1, 2, 3and 4.

FIG. 1 represents a catalyst slurry preparation system according to theinvention comprising two mud pots 2 containing concentrated catalystslurry. Also according to the invention, only one mud pot 2 may bepresent. Catalyst slurry can be prepared by providing dry catalyst froma catalyst supply vessel (not shown) to said mud pots 2. Both mud potsare provided with an injection valve 32 for the addition of diluent intothe mud pots 2. Conduits 6, 7 and 15 connect the mud pots 2 with amixing vessel 3, wherein the catalyst slurry is diluted to aconcentration suitable for use in a polymerization reaction. The conduit6 for transferring said catalyst slurry from a first mud pot 2 to amixing vessel 3 is interchangeable with a second conduit 7 fortransferring said catalyst slurry from a second mud pot 2 to a mixingvessel 3 through lines 8 connecting said first 6 with said second 7conduit. Such interconnection 8 permits, in case of interruption oftransfer through one conduit 6, to discharge the catalyst slurry to themixing vessel 3 through a second conduit 7. Conduit 6 and 7 may beprovided with catalyst slurry feeders 9 for dosing the feed of catalystslurry from the mud pots 2 to the mixing vessel 3 and injection valves24 for the injection of diluent. The mixing vessel 3 is provided withmixing means 25. The mixing means comprises at least two double-bladedhubs 125,225. Preferably the mixing means 25 is a rotatable magneticdrive impeller system 25. The diluted slurry is then pumped throughconduit 4 in a polymerization reactor 1. To that end the conduit 4 whichconnects the mixing vessel 3 to a polymerization reactor 1 is providedwith pumping means 5. The conduit 4 may be further provided with diluentflushing means 30, 33 and flow and concentration measuring means 10,such as e.g. a coriolis flow meter. These flow and concentrationmeasuring means 10 can be provided upstream and downstream from saidpumps 5. Diluent flushing means 30, 33 enable to flush diluent such asisobutane through the conduit 4 and to keep the conduit 4 and thepumping means 5 unplugged. Conduit 4 may be further provided with aconduit and valve 31 for by-passing the pump 5. The conduits 4 fortransferring catalyst slurry into to the reactor may also be equipped byone or more valves, preferably piston valves 22. The piston valves 22are capable of sealing the orifice by which the conduit 4 is connectedto the reactor 1.

FIG. 2 represents a cylindrical mixing vessel 3 of a catalyst slurrypreparation system according to the invention comprising a top part 118,a bottom part 119, and a rotatable magnetic drive impeller system 25which is actuated by an electromotor 120. The impeller system 25comprises an agitator shaft 117 which is positioned along a central axisof the mixing vessel 3 and extends through the top part of the mixingvessel 3 where it is held by an agitator bearing unit 121, comprisingthe magnetic elements (not shown). The agitator bearing unit 121 isconnected to the electromotor 120. The electromotor generates energywhich is translated by the agitator bearing unit 121 into rotation ofthe agitator shaft 117. Two double-bladed hubs 125, 225 are slidablyfixed to the agitator shaft 117. The top double-bladed hub 125 is fixedto the agitator shaft 117 at about 60% 133 of the length of the mixingvessel 131 from the top of the mixing vessel 118. The bottomdouble-bladed hub 225 is fixed to the agitator shaft 117 at about 85%134 of the length of the mixing vessel 131 from the top of the mixingvessel 118. The distance 135 between the two double-bladed hubs 125, 225is about 27.5% of the length of the mixing vessel 131. The blade span136 of each double-bladed hub 125, 225 is about 40% of the diameter ofthe mixing vessel 132. The mixing vessel 3 further comprises 3 baffles124, 224 (and 324, not shown on FIG. 2) which are positionedlongitudinally along the inner side wall of the mixing vessel 3 andextend radially inward from the inner wall of the mixing vessel 3 over awidth 137 of about 15% of the diameter of the mixing vessel 132. Themixing vessel also comprises an outlet 123 for diluted catalyst slurryat the bottom 119 of the mixing vessel 3.

The direction of rotation 140 of the impeller system 25 is such that theflow 141 of the catalyst slurry is directed predominantly axial towardsthe bottom of the mixing vessel 119.

FIG. 3 represents an enlarged view of an embodiment of double-bladed hub125 comprising two blades 126, 226 which are symmetrically fixed on thehub 125. The double-bladed hub 125 is slidably fixed to the agitatorshaft 117. Each blade 126, 226 comprises a root portion 127, 227 and atip portion 128, 228 with in between an upper side edge 129, 229 and alower side edge 130, 230. The upper side edge 129, 229 extendslongitudinally from the root portion 127, 227 and inclines concavelytowards the tip portion 128, 228. The lower side edge 130, 230 isconcave. The tip portion 128, 228 inclines convexly between the lowerside edge 130, 230 and the upper side edge 129, 229. Arrow 140 shown thedirection of rotation.

FIG. 4 represents a view along the central axis between the root portion127, 227 and the tip portion 128, 228 of a blade 126, 226 of adouble-bladed hub 125, 225. The double-bladed hub 125,225 is slidablyfixed to the agitator shaft 117 of the rotatable magnetic drive impellersystem 25. The illustrated pitch angle (α) of each blade is about 75°.Arrow 140 shown the direction of rotation.

FIG. 6 represents a schematic cross-view of some elements of therotatable magnetic drive impeller system 25. The rotatable magneticdrive impeller system 25 comprises the agitator shaft 117 which ispositioned along a central axis of the mixing vessel 3 (only partiallyshown) and extends through the top part of the mixing vessel 3 where itis held by an agitator unit 121, comprising the magnetic elements310,320. The agitator shaft 117, magnetic elements 310 and inner rotor340 are supported by bearings positioned on the lower housing 400. Theagitator unit 121 is connected to the electromotor (not shown) byconnection using a shaft and/or gear box (both not shown) via the topflange 370 of the unit 12′1. The electromotor generates energy which istranslated by the agitator unit 121 into rotation (arrows 140) of theagitator shaft 117 which is provided with at least two double-bladedhubs 125, 225. The agitator unit 121, comprises a magnet couplinghousing 300 and a lower housing 400. The magnetic coupling housing 300is connected to lower housing 400 via the bottom flange 360 of themagnetic coupling housing 300 which is connected with bolts 440 to thetop flange 420 of the lower housing 400. The agitator unit 121 isconnected to the mixing vessel 3 via the bottom flange 410 of the lowerhousing 400 which is connected with bolts 430 to the top flange 35 ofthe mixing vessel 3. The magnetic coupling housing 300 comprises aninner rotor 340 disposed coaxially on shaft 117, and an outer rotor 350.The outer rotor 350 of the coupling housing 300 is operably connected(not shown) to the electromotor. Magnetic coupling housing 300 hascircumferentially disposed arrays of permanent magnets 320, 310 in itsinner and outer rotors 340 and 350, respectively. The magnets in theinner rotor 340 are aligned with the magnets in the outer rotor 350 butare polarized oppositely. The magnetic coupling housing 300 has aconfinement shell 330 (also known as a separator, separating member, orcontainment shell) having a flange 370 which is bolted in the assemblyby bolts 440. The confinement shell 330 isolates the inner rotor 340 andprevents leakage of the mixing tank.

In yet another aspect, the invention relates to a method for preparing acatalyst slurry, in particular a diluted catalyst slurry, by means ofthe catalyst slurry preparation system as described herein. Inparticular, the present invention also relates to a method for preparinga diluted catalyst slurry or a method for diluting a catalyst slurry, ina catalyst slurry preparation system as described herein, by the stepsof

(a1) feeding a concentrated catalyst slurry into the mixing vessel;

(a2) diluting the concentrated catalyst slurry in a suitable amount ofdiluent in the mixing vessel thereby obtaining a diluted catalyst slurryhaving a concentration suitable for use in an ethylene polymerizationreaction; and

(a3) mixing the diluted catalyst slurry in the mixing vessel.

In a preferred embodiment, a method is provided for preparing catalystslurry in a catalyst slurry preparation system comprising a rotatableimpeller system 25 as described herein, wherein the catalyst slurrycomprises a solid particulate catalyst and a liquid hydrocarbon diluent,the method comprising the steps of:

(a1) preparing concentrated catalyst slurry in one or more mud pots 2;

(a2) transferring the concentrated catalyst slurry from the one or moremud pots 2 to the mixing vessel 3 through one or more conduits 6, 7; and

(a3) diluting the concentrated catalyst slurry in a suitable amount ofdiluent in the mixing vessel 3 thereby obtaining a diluted catalystslurry having a concentration suitable for use in an ethylenepolymerization reaction; and

(a4) mixing the diluted catalyst slurry in the mixing vessel 3 byrotation of the impeller system 25 according to the invention.

Preferably, a method is provided wherein the concentration of the solidparticulate catalyst in the liquid hydrocarbon diluent in the mixingvessel is between 0.1% and 10% by weight, preferably between 0.2% and 5%by weight, and most preferably between 0.3% and 3% by weight.

In another embodiment a method is provided wherein said solidparticulate catalyst has an average diameter comprised between 1 μm and100 μm, preferably between 5 μm and 100 μm, more preferably between 10μm and 100 μm or between 5 μm and 50 μm, even more preferably between 15μm and 50 μm and most preferably about 40 μm.

In yet another preferred embodiment, a method is provided wherein saidimpeller system rotates between 50 rpm (rotations per minute) and 1000rpm, preferably between 150 rpm and 450 rpm, more preferably between 200rpm and 350 rpm, and even more preferably about 320 rpm.

The catalyst slurry preparation system as described herein can be usedfor the preparation of a diluted catalyst slurry or for diluting acatalyst slurry, said catalyst slurry comprising a solid particulatecatalyst and a liquid hydrocarbon diluent, wherein said particulatecatalyst is a metallocene catalyst, a chromium catalyst or aZiegler-Natta catalyst. In an embodiment, the particulate catalyst isimmobilized on a support, preferably a silica support.

In a further embodiment, the catalyst slurry preparation system asdescribed herein can be used for bringing settled or precipitatedcatalyst back into suspension.

In a preferred embodiment of the present invention, said catalyst is ametallocene catalyst. The term “metallocene catalyst” is used herein todescribe any transition metal complexes consisting of metal atoms bondedto one or more ligands. The metallocene catalysts are compounds of GroupIV transition metals of the Periodic Table such as titanium, zirconium,hafnium, etc., and have a coordinated structure with a metal compoundand ligands composed of one or two groups of cyclopentadienyl, indenyl,fluorenyl or their derivatives. Use of metallocene catalysts in thepolymerization of olefins has various advantages. Metallocene catalystshave high activities and are capable of preparing polymers with enhancedphysical properties. The key to metallocenes is the structure of thecomplex. The structure and geometry of the metallocene can be varied toadapt to the specific need of the producer depending on the desiredpolymer. Metallocenes comprise a single metal site, which allows formore control of branching and molecular weight distribution of thepolymer. Monomers are inserted between the metal and the growing chainof polymer.

In a preferred embodiment, the metallocene catalyst has a generalformula (I) or (II):(Ar)₂MQ₂  (I); orR″(Ar)₂MQ₂  (II)wherein the metallocenes according to formula (I) are non-bridgedmetallocenes and the metallocenes according to formula (II) are bridgedmetallocenes;wherein said metallocene according to formula (I) or (II) has two Arbound to M which can be the same or different from each other;wherein Ar is an aromatic ring, group or moiety and wherein each Ar isindependently selected from the group consisting of cyclopentadienyl,indenyl, tetrahydroindenyl or fluorenyl, wherein each of said groups maybe optionally substituted with one or more substituents eachindependently selected from the group consisting of halogen, ahydrosilyl, a SiR₃ group wherein R is a hydrocarbyl having 1 to 20carbon atoms, and a hydrocarbyl having 1 to 20 carbon atoms and whereinsaid hydrocarbyl optionally contains one or more atoms selected from thegroup comprising B, Si, S, O, F, Cl and P;wherein M is a transition metal M selected from the group consisting oftitanium, zirconium, hafnium and vanadium; and preferably is zirconium;wherein each Q is independently selected from the group consisting ofhalogen; a hydrocarboxy having 1 to 20 carbon atoms; and a hydrocarbylhaving 1 to 20 carbon atoms and wherein said hydrocarbyl optionallycontains one or more atoms selected from the group comprising B, Si, S,O, F, Cl and P; andwherein R″ is a divalent group or moiety bridging the two Ar groups andselected from the group consisting of a C₁-C₂₀ alkylene, a germanium, asilicon, a siloxane, an alkylphosphine and an amine, and wherein said R″is optionally substituted with one or more substituents eachindependently selected from the group consisting of halogen, ahydrosilyl, a SiR₃ group wherein R is a hydrocarbyl having 1 to 20carbon atoms, and a hydrocarbyl having 1 to 20 carbon atoms and whereinsaid hydrocarbyl optionally contains one or more atoms selected from thegroup comprising B, Si, S, O, F, Cl and P.

The term “hydrocarbyl having 1 to 20 carbon atoms” as used herein isintended to refer to a moiety selected from the group comprising alinear or branched C₁-C₂₀ alkyl; C₃-C₂₀ cycloalkyl; C₆-C₂₀ aryl; C₇-C₂₀alkylaryl and C₇-C₂₀ arylalkyl, or any combinations thereof. Exemplaryhydrocarbyl groups are methyl, ethyl, propyl, butyl, amyl, isoamyl,hexyl, isobutyl, heptyl, octyl, nonyl, decyl, cetyl, 2-ethylhexyl, andphenyl. Exemplary halogen atoms include chlorine, bromine, fluorine andiodine and of these halogen atoms, fluorine and chlorine are preferred.

Illustrative examples of metallocene catalysts comprise but are notlimited to bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂),bis(cyclopentadienyl)titanium dichloride (Cp₂TiCl₂),bis(cyclopentadienyl)hafnium dichloride (Cp₂HfCl₂);bis(tetrahydroindenyl)zirconium dichloride, bis(indenyl)zirconiumdichloride, and bis(n-butyl-cyclopentadienyl)zirconium dichloride;ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride,ethylenebis(1-indenyl)zirconium dichloride, dimethylsilylenebis(2-methyl-4-phenyl-inden-1-yl)zirconium dichloride, diphenylmethylene(cyclopentadienyl)fluoren-9-yl)zirconium dichloride, anddimethylmethylene[1-(4-tert-butyl-2-methyl-cyclopentadienyl)](fluoren-9-yl)zirconium dichloride.

The catalysts can be provided on a solid support. The support should bean inert solid, which is chemically unreactive with any of thecomponents of the conventional metallocene catalyst. The support orcarrier is an inert organic or inorganic solid, which is chemicallyunreactive with any of the components of the conventional metallocenecatalyst. Suitable support materials for the supported catalyst of thepresent invention include solid inorganic oxides, such as silica,alumina, magnesium oxide, titanium oxide, thorium oxide, as well asmixed oxides of silica and one or more Group 2 or 13 metal oxides, suchas silica-magnesia and silica-alumina mixed oxides. Silica, alumina, andmixed oxides of silica and one or more Group 2 or 13 metal oxides arepreferred support materials. Preferred examples of such mixed oxides arethe silica-aluminas. Most preferred is silica. The silica may be ingranular, agglomerated, fumed or other form. The support is preferably asilica compound. In a preferred embodiment, the metallocene catalyst isprovided on a solid support, preferably a silica support.

In a preferred embodiment, a polymerization catalyst applied in thepresent polymerization process is a supported metallocene-alumoxanecatalyst consisting of a metallocene and an alumoxane which are bound ona porous silica support.

In another embodiment of the present invention, said catalyst is achromium catalyst. The term “chromium catalysts” refers to catalystsobtained by deposition of chromium oxide on a support, e.g. a silica oraluminum support. Illustrative examples of chromium catalysts comprisebut are not limited to CrSiO₂ or CrAl₂O₃.

In another embodiment of the present invention, said catalyst is aZiegler-Natta catalyst. The term “Ziegler-Natta catalyst” or “ZNcatalyst” refers to catalysts having a general formula M¹X^(n), whereinM¹ is a transition metal compound selected from group IV to VII, whereinX is a halogen, and wherein n is the valence of the metal. Preferably,M¹ is a group IV, group V or group VI metal, more preferably titanium,chromium or vanadium and most preferably titanium. Preferably, X ischlorine or bromine, and most preferably, chlorine. Illustrativeexamples of the transition metal compounds comprise but are not limitedto TiCl₃, TiCl₄. Suitable ZN catalysts for use in the invention aredescribed in U.S. Pat. No. 6,930,071 and U.S. Pat. No. 6,864,207, whichare incorporated herein by reference.

In an aspect, the invention also relates to a process for preparing aparticulate polyethylene product in a polymerization loop reactor,comprising the steps of:

-   (a) feeding ethylene monomer, a liquid hydrocarbon diluent,    optionally hydrogen, and optionally olefin co-monomer into said loop    reactor;-   (b) feeding a catalyst slurry prepared according to a method of the    invention as described herein into said loop reactor;-   (c) polymerizing said monomer and said optionally co-monomer to    produce a polyethylene slurry in said diluent in said loop reactor;-   (d) allowing said polyethylene slurry to settle into one or more    settling legs connected to said loop reactor;-   (e) discharging the settled polyethylene slurry from said one or    more settling legs out of said loop reactor.

The following non-limiting example illustrates the invention.

EXAMPLES Example 1

The present example illustrates the use of a catalyst slurry preparationsystem according to an embodiment of the invention for preparing dilutedcatalyst slurry. The polymerization catalyst comprises a metallocenecatalyst immobilized on a porous silica support. The metalloceneconsists in particular of ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride. The median particle diameter of the polymerizationcatalyst is 40 μm. The polymerization catalyst can be used to prepare aparticulate polyethylene resin in a loop reactor.

The characteristics of the mixing vessel and the process characteristicsfor the present example for preparing diluted catalyst slurry are givenin Table 1. The flow rate measures how much catalyst slurry is hourlyoutputted from the mixing vessel, and is thus a measure for catalystresidence time in the mixing vessel as well as catalyst turnover time inthe mixing vessel.

TABLE 1 volume of mixing vessel (I) 500 average mixing speed (rpm) 325flow rate (l/h) 200 average catalyst concentration (wt %) 0.42

The concentration of polymerization catalyst (wt % on Y-ordinate) at theoutlet of the mixing vessel, i.e. before feeding to the polymerizationreactor, was measured over a time span of 2 days (on X-ordinate). Theresults are illustrated in FIG. 5. As can be seen, the concentration isrelatively constant over time, indicating adequate mixing in the mixingvessel, resulting in homogeneous catalyst slurry with a mainlyconsistent catalyst concentration which can be fed to the polymerizationreactor.

Example 2

A mixing tank comprising a magnetic actuated agitator shaft driven bymagnetic drive impeller system as schematically illustrated in FIGS. 2and 6 was used to prepare a catalyst slurry. The catalyst slurrycomprised 0.5% by weight of a metallocene catalyst immobilized on aporous silica support in an isobutane diluent. The metallocene consistsin particular of ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconiumdichloride. The mixing tank was operated liquid full. Due to themagnetic coupling, the vessel containing the agitator was hermeticallyclosed and the catalyst slurry could be prepared under liquid fullconditions without any leakage.

Comparatively, a mixing tank comprising an agitator with a mechanicalseal could not be used liquid full without observing leakage.

The invention claimed is:
 1. A catalyst slurry preparation system forthe preparation of a diluted catalyst slurry comprising a solidparticulate catalyst and a liquid hydrocarbon diluent, wherein saidcatalyst slurry preparation system comprises: a mixing vessel comprisinga top part, a bottom part and a rotatable impeller system which isactuated by a motor; wherein the rotatable impeller system comprises amagnetic actuated agitator shaft which is positioned along alongitudinal axis of said mixing vessel and extends through said toppart of said mixing vessel and comprises at least two double-bladed hubswhich are fixed to said magnetic actuated agitator shaft, wherein eachdouble-bladed hub is a hub comprising two blades; wherein each blade ofeach double-bladed hub is an airfoil comprising a root portion attachedto the hub and a radially outwardly disposed tip portion with an upperside edge therebetween, wherein the upper side edge faces the top partof said mixing vessel, and wherein each blade comprises a lower sideedge facing said bottom part of said mixing vessel; and wherein said tipportion inclines convexly toward said top part of said mixing vesselbeyond said upper side edge, wherein said upper side edge extendslaterally from said root portion to said tip portion and inclinesconcavely toward said tip portion, and wherein said lower side edge isconcave.
 2. The catalyst slurry preparation system according to claim 1,wherein said mixing vessel further comprises one or more baffles,wherein said one or more baffles are fixed longitudinally along an innerwall of said mixing vessel, whereby said one or more baffles extendradially inward.
 3. The catalyst slurry preparation system according toclaim 2, wherein said one or more baffles extend radially inward over adistance of between 10% and 20% of a diameter of said mixing vessel. 4.The catalyst slurry preparation system according to claim 1, whereinblades of each double-bladed hub are symmetrically positioned about saiddouble-bladed hub and have a pitch angle (α) between 65° and 75°.
 5. Thecatalyst slurry preparation system according to claim 1, wherein a firstdouble-bladed hub is fixed to said magnetic actuated agitator shaft in alower half of said mixing vessel.
 6. The catalyst slurry preparationsystem according to claim 5, wherein a second double-bladed hub is fixedto said magnetic actuated agitator shaft in a lower quarter of saidmixing vessel.
 7. The catalyst slurry preparation system according toclaim 6, wherein a distance between said first and said seconddouble-bladed hub is between one half to one third of a length of saidmagnetic actuated agitator shaft.
 8. The catalyst slurry preparationsystem according to claim 1, wherein each double-bladed hub has a bladespan of between 30% and 50% of a diameter of said mixing vessel.
 9. Thecatalyst slurry preparation system according to claim 1, furthercomprising one or more mud pots suitable for containing a concentratedcatalyst slurry, wherein each mud pot is operably connected to saidmixing vessel.
 10. The catalyst slurry preparation system according toclaim 1, wherein said rotatable impeller system is adapted to rotate ata speed of between 50 and 1000 rpm.
 11. The catalyst slurry preparationsystem according to claim 1, wherein said rotatable impeller system isadapted to rotate at a speed of between 150 and 450 rpm.
 12. Thecatalyst slurry preparation system according to claim 1, wherein saidmixing vessel is cylindrical.
 13. A catalyst slurry preparation systemcomprising: a mixing vessel comprising a top part, a bottom part and animpeller system that is configured to be actuated by a motor; whereinthe impeller system comprises a magnetic actuated agitator shaft that ispositioned along a longitudinal axis of the mixing vessel and extendsthrough the top part of the mixing vessel, wherein at least twodouble-bladed hubs are fixed to the magnetic actuated agitator shaft andwherein each double-bladed hub is a hub comprising two blades; and oneor more mud pots suitable for containing a concentrated catalyst slurry,wherein each mud pot is operably connected to said mixing vessel;wherein each blade of each double-bladed hub is an airfoil comprising aroot portion attached to the hub and a radially outwardly disposed tipportion with an upper side edge therebetween, wherein the upper sideedge faces the top part of said mixing vessel, and wherein each bladecomprises a lower side edge facing said bottom part of said mixingvessel.
 14. The catalyst slurry preparation system according to claim13, wherein said tip portion inclines convexly toward said top part ofsaid mixing vessel beyond said upper side edge, wherein said upper sideedge extends laterally from said root portion to said tip portion andinclines concavely toward said tip portion, and wherein said lower sideedge is concave.
 15. The catalyst slurry preparation system of claim 13,wherein the mixing vessel is connected to a polymerization reactor. 16.The catalyst slurry preparation system of claim 15, wherein the mixingvessel comprises an inlet for concentrated catalyst slurry and diluent,and an outlet for diluted catalyst slurry, and wherein the at least twodouble-bladed hubs direct flow to the bottom part of the mixing vesselaway from the inlet and towards the outlet.
 17. The catalyst slurrypreparation system of claim 13, wherein each blade of each double-bladedhub is a flat plate airfoil.